1. Field of the Invention
The present invention relates to an electrically conductive bonding material, a method of bonding with the same, and a semiconductor device bonded with the same. For example, the invention relates to a bonding material and a method of bonding used in a semiconductor module.
2. Background Art
In a non-insulated semiconductor device used in inverters and the like as a power semiconductor device, an anchoring member for attaching a semiconductor element functions as an electrode of the semiconductor device. For example, in a semiconductor device having a power transistor mounted on an anchoring member with a Sn—Pb solder, the anchoring member (base member) is a collector electrode of the power transistor. In operating the semiconductor device, the collector electrode carries an electric current of several or more amperes, resulting in heating the transistor chip. In order to prevent property destabilization or service life reduction due to the heat generation, a good heat dissipation capacity and a long-term reliability (heat resistance) have to be secured in the soldered portion. A material with high heat dissipation is required for securing the heat resistance and heat dissipation performance in a soldered portion.
Similarly, in an insulated semiconductor device, for safe and stable operation of a semiconductor element, developed heat during operating period of a semiconductor device needs to be efficiently dissipated to the exterior of the semiconductor device and reliability of bonding in a soldered portion needs to be secured. For example, in an environment, such as an automotive engine compartment, frequently exposed to high temperatures, the reliability of a bonding portion must be sufficiently high.
Although a bonding material having a high melting point may be used for enhancing heat resistance of a bonding portion, bonding temperature is inevitably raised in this case. A high bonding temperature may cause an anchoring member to get damaged by heat or may cause a warp of an entire semiconductor device due to a difference in stress between a substrate and the anchoring member during a cooling period after bonding. That means heat resistance of a bonding material needs to be enhanced while bonding temperature needs to be lowered.
For example, as described in JP Patent Publication (Kokai) No. 2003-309352, an electrically conductive bonding material which is an electrically conductive composition containing a granular silver compound is known as a bonding material having a high heat dissipation capacity and a high reliability.
In JP Patent Publication (Kokai) No. 2005-129303, a method for sintering particles having sizes in a range of 1 μm to 100 μm in a bonding layer is disclosed.
It is known that a metal particle having a particle size of not greater than 100 nm with few constituent atoms has a sharply increased ratio of surface area to volume of the particle, so that the sintering temperature is substantially reduced compared to that of a bulk form (resulting in increased sintering performance or increased surface energy). For example, JP Patent Publication No. 2004-107728 discloses that metal particles having an average particle size of not greater than 100 nm coated with an organic material are used as bonding material and bonding is performed by sintering the metal particles with each other through decomposition of the organic material by heat, utilizing this low-temperature sintering effect. According to this method of bonding, the metal particles after bonding are transformed into bulk metal and moreover the bonding interface is connected by metal bonding, so that the interface has very high heat resistance and reliability, and a high heat dissipation capacity.